1. Field of the Invention
This invention relates to lithographic processes, and in particular, to lithographic processes involving device fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing light is coated onto a substrate, e.g., a silicon wafer, that is being processed to form a plurality of devices. The coating material, i.e., the resist, is then subjected to light that has been passed through a mask material so that the light reaching the resist corresponds to a desired pattern that is to be transferred into the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron lithography.
Improved resolution with a blanket exposure is achievable by using deep ultraviolet or x-ray light. X-ray exposure generally has a potential for better resolution than exposure with deep ultraviolet light, but has not been studied as extensively. One approach to a photoresist sensitive to deep ultraviolet radiation employs a compound that produces an acid moiety upon irradiation with such radiation together with a polymer that reacts with the generated acid. Typical acid generator/acid sensitive polymer combinations include an onium salt as the photosensitive acid generator and a polymer such as poly(4-t-butoxycarbonyloxystyrene) as the polymer having a reactive substituent. (See Ito, et al. U.S. Pat. No. 4,491,628 dated Jan. 1, 1985.) Such systems are generally referred to as chemical amplification systems since the production of one molecule of acid by actinic radiation induces a reaction in a plurality of molecules in the acid sensitive polymer.
To enhance the sensitivity of acid generator/polymer combination one proposal employs a polymer including both a substituent sensitive to acid and a moiety present in the polymer chain that induces upon irradiation chain scission with associated decrease in molecular weight. As described by R. G. Tarascon, et al, Proceedings of Regional Technical Conference on Photopolymers, Principles, Processes and Materials, Mid Hudson Section, Society of Plastic Engineers, Oct. 30 to Nov. 2, 1988, Ellenville, N.Y., page 11, one such combination includes an acid generator and a polymer having a sulfone moiety in the backbone. Although this combination gives excellent results, the presence of more than one material in the resist does add processing and preparation complications.